The widely accepted free radical theory of aging (FRTA) proposes that aging results from the accumulation of oxidative damage caused by reactive oxygen species (ROS) generated during normal metabolism (figure 1). However, recent work in the worm Caenorhabditis elegans has indicated that the relationship between ROS and life span is more complex than anticipated. Superoxide dismutase (SOD) is an enzyme that decreases the levels of ROS, but decreasing the antioxidant defense through the deletion of SOD genes, individually or in combination, does not decrease life span. In fact, quintuple-mutant worms lacking all five sod genes live as long as wild-type worms despite a markedly increased sensitivity to oxidative stress (figure 2). Thus, it appears that while oxidative damage increases with age, it does not cause aging.
Figure 1. The Free Radical Theory of Aging. The free radical theory of aging (FRTA) proposes that reactive oxygen species (ROS) are the cause of aging. ROS are highly reactive oxygen containing molecules and are generated by normal metabolism when energy is produced in the mitochondria. ROS can cause damage to the basic building blocks of the cell including DNA, protein and lipids. This damage, which is called oxidative damage, has been shown to increase with age. The FRTA proposes that the accumulation of this damage eventually results in cellular and tissue dysfunction, increasing the probability of death and thereby causing aging.
Figure 2. Loss of superoxide dismutase activity does not decreased lifespan. To test the effect of antioxidant defense on lifespan, we compared the lifespan of superoxide dismutase (SOD) quintuple mutant worms to wild-type worms. Despite having no SOD activity (A) and having markedly increased sensitivity to oxidative stress (B), SOD quintuple mutant worms do not show decreased lifespan (C). This indicates that oxidative stress resistance and lifespan can be experimentally dissociated.
Recent evidence suggests that increased levels of superoxide can act as a pro-survival signal that leads to increased longevity. This is demonstrated by life-span increases following the deletion of the mitochondrial gene sod-2 and the treatment of wild-type worms with the superoxide generator paraquat (figure 3). The fact that sod quintuple-mutant worms have a normal life span despite their increased sensitivity to oxidative stress suggests a balance between the pro-survival signaling and the toxic effects of superoxide.
Figure 3. Increasing superoxide levels or decreasing superoxide detoxification increases lifespan. (A) Worms with a deletion in the mitochondrial superoxide dismutase (sod) gene, sod-2, show increased lifespan compared to wild-type worms. (B) Similarly, increasing superoxide levels in wild-type worms with the superoxide generating compound paraquat also results in increased lifespan.
Thus, one of the main goals of this work is to elucidate the mechanism by which superoxide-mediated pro-survival signaling leads to increased longevity (figure 4). We explore how increased levels of superoxide trigger the signal, how the signal is transmitted, and which changes the signal introduces lead to increased life span. These experiments use a combination of genetic mutants and RNA interference to gain insight into the signaling mechanism.
Figure 4. Superoxide-mediated pro-survival signalling model. In this model, increased levels of superoxide trigger a pro-survival signal that engages mechanisms intended to survive the superoxide stress. In addition to promoting survival against the superoxide stress, these mechanisms result in increased lifespan.
The greatest risk factor for developing Parkinson’s disease (PD) is advanced age. Even individuals with the inherited forms of PD live decades without exhibiting symptoms or neuronal loss, despite the fact that the disease-causing mutation is already present at birth. This suggests that changes taking place during normal aging make cells more susceptible to the mutations implicated in PD. This conclusion is supported by the fact that the onset of the disease in animal models is proportional to the life span of the organism and is not related to chronological time. Moreover, several changes known to take place during the aging process have been shown to affect functions implicated in the pathogenesis of PD. These include protein aggregation, increased oxidative stress, decreased mitochondrial function, dysfunction of the proteasome, and impairment of autophagy. Therefore, we propose that by targeting specific age-related changes we can develop new treatment strategies. Our approach is supported by studies showing the benefit of specific interventions that increase life span in animal models of PD.
The main goals of this work will be to determine whether genes that extend life span are beneficial in the treatment of worm models of PD and to determine whether processes that show decreased function with age specifically exacerbate PD-like features in worm models of PD. This will be accomplished through two complementary approaches. First, genetic crosses will be used to generate double mutants. Second, RNA interference by feeding will be used to specifically knock down the expression of the gene of interest. In both cases, the health of the resulting worms will be compared with that of PD control worms to determine whether the aging gene has had an effect on the disease-like abnormalities.
By examining the role of aging in PD, this project will provide new insight into the mechanism underlying PD. This knowledge will provide novel therapeutic targets that may lead to an effective treatment.